The invention relates generally to detecting and tracking aircraft using coded reply signals transmitted from aircraft transponders. More specifically, the invention relates to such detecting and tracking under conditions in which multiple target reply signals are received during a receive interval following an interrogation and need to be matched to multiple target signatures or tracks.
Air traffic control and safety are ongoing concerns in commercial and military aviation. Particularly significant concerns are traffic alert and collision avoidance between aircraft either in route between or in the vicinity of landing fields. Ever increasing air traffic demands have resulted in governmental regulations that require commercial carriers to equip planes with active interrogation systems that can determine the presence and threat of nearby aircraft called targets. The particular system mandated by the government depends on the aircraft size. Large commercial aircraft that carry over 30 passengers are being equipped with an active traffic and collision avoidance system (TCAS II) that not only detects and displays nearby aircraft, but also alerts the crew as to impending collisions, and also provides resolution advisories such as audible instructions to the pilot to pull up or down, maintain level or climb rate and so forth. This system, however, is very complex and expensive and therefore has not been mandated for smaller aircraft.
For aircraft that carry up to 30 passengers, governmental regulations require such aircraft be equipped with an active interrogation system (TCAS I) that detects nearby aircraft, determines and displays range, bearing and altitude of such aircraft relative to the interrogating plane, and tracks such aircraft within a prescribed range and issues an audible alert to the crew as to impending collisions. Although the operational performance of the TCAS I system appears less complex than TCAS II, numerous problems arise that make a cost effective system difficult to realize.
The Federal Aviation Administration (FAA) specifies that the TCAS I active interrogation systems use air traffic control radar beacon system (ATCRBS) signals. These ATCRBS interrogation signals are high frequency pulse amplitude modulated signals at 1030 Megahertz. The reply signals are also pulse amplitude modulated but at a carrier frequency of 1090 Megahertz. In TCAS I, the reply and interrogation signals are transmitted from an interrogation aircraft to other aircraft in the vicinity thereof, and these other aircraft respond to the interrogations via a transponder located on the aircraft.
The interrogation and reply signal waveforms are specified by the FAA. The information contained in the reply signal depends on the type of interrogation (e.g. Mode A, Mode C) and the transponder equipment that the interrogated aircraft has available for responding. For TCAS I, the interrogation mode is Mode C, and the Mode C reply signal from the aircraft transponder consists primarily of encoded altitude data. The altitude data is encoded using binary logic states or bits arranged in four digit octal codes (i.e. each octal altitude code has twelve data bits with each octal digit defined by three data bits). The reply signal data bits are transmitted within a pair of framing pulses called bracket pulses that indicate (for purposes of TCAS I) the beginning and end of an altitude code reply signal from a particular aircraft responding to an interrogation.
A TCAS I system is specified based on the use of these ATCRBS Mode C reply signal waveforms. Thus, an interrogating aircraft may transmit an interrogation signal at 1030 MHz, and then will "listen" for Mode C reply signals from all aircraft capable of responding by transmitting the bracket pulses and altitude encoded data pulses. Some aircraft are not equipped to reply with altitude data (non-altitude reporting, or NAR) and hence only transmit the bracket pulses. Under TCAS I, aircraft within a range of about 34 nautical miles will reply to a Mode C interrogation.
In addition to having to detect and decode reply signals, a TCAS I compatible system must be able to track responding aircraft so as to provide traffic alert information. This can be done, for example, by means of a visual display that shows in a real time manner the movement and altitude of aircraft that are being tracked by the system. This tracking is accomplished by decoding the reply signals and deciding which reply signals can be correlated over a number of update sequences to provide a target signature. A target signature can be thought of as a number of tracked parameters decoded from the reply signals that indicate the most current estimate of position and movement of a target relative to the interrogating aircraft, including range, bearing and altitude parameter tracks. Once a target signature has been established, it must be periodically updated, preferably during each update period following an interrogation, in order to maintain current information on the target.
Reply signal decoding, tracking and matching is less complicated in low traffic areas because there will tend to be only a few reply signals received during each listening period following an active interrogation by the interrogating aircraft. Typically, only a few aircraft will be tracked, and only a few reply signals will be received that need to be matched with those target signatures to update and maintain tracking information.
In higher traffic areas, however, numerous problems arise that make target tracking much more difficult. In TCAS I, for example, an interrogation sequence includes a number of transmissions from the interrogating aircraft, called a whisper/shout sequence, that is intended to reduce the number of replies received at the same time. However, the TCAS I whisper/shout sequence cannot prevent multiple reply signals from being returned from multiple targets during the same time interval. Therefore, not only must a TCAS I system be able to detect and separate these multiple and overlapping reply signals, but it must also then be able to match correctly each reply with its target signature (assuming that a reply is from an aircraft that has an established signature). In other words, the system must be able to perform an accurate data association between newly received reply signals and established target signatures. When multiple targets are in the vicinity of the interrogating aircraft, this data association becomes very important so that the target tracks can be updated with the most accurate and current information available to the system.
In addition to reply signals received from active interrogation transmissions from an interrogating aircraft, the interrogating aircraft may also receive and detect passive reply signals. These passive reply signals are issued by target aircraft in response to radar interrogations from ground based air traffic control, for example. In addition to providing range, bearing and altitude information, these passive replies may also include differential azimuth and target aircraft identification information. This additional information can be useful for enhancing target tracking and update.
Because target aircraft move constantly with respect to an interrogating aircraft, an important aspect of a TCAS I system is the use of target signature information to predict the position and movement of the targets. This enables the system to more accurately match the various reply signals to established signatures. An important parameter that is predicted is target altitude. Typically, target altitude will not change drastically between interrogation intervals used in TCAS I. Therefore, once a target signature has been established, altitude is a useful parameter for matching a reply signal to update its signature. Although not all targets will be equipped to respond with an altitude code (such aircraft being referred to herein as non-altitude reporting or NAR), those that do transmit altitude codes will occasionally have their altitude codes become garbled during the transmission, receiving and decoding process, particularly in high traffic areas where there are many overlapping reply signals. Target signature update can be enhanced if such garbled altitude codes can be repaired.
The objectives exist, therefore, for a traffic alert and collision avoidance system that can accurately associate or correlate target reply signals with established target signatures. As part of this process, such a system should be able to repair garbled codes to enhance target signature updates.